Most people find long spells of hot weather irritating or fatiguing. A growing body of research shows climate change may actually be causing psychological distress and other health problems.

Maithili Ramachandran, a public policy scholar at the University of California, Riverside, is part of a team of researchers comparing the relationship of heat waves, zip codes, and health symptoms. She said they found higher incidences of stress, anxiety and depression during hot periods, even after controlling for socioeconomic factors. The study authors controlled for gender, age and income.

They found the correlation is strongest during strings of warm nights. Ramachandran said that’s because the cool of evening helps the body rest and recuperate.

“Night-time heat waves are an additional stress factor because they sometimes coincide with daytime heat waves and then it’s just too much,” she said.

They also noted that pregnancies were affected by heat, especially if spikes occurred during months that are typically cold.

“We found that in the second and third trimesters this kind of extreme heat, like days over 90 degrees fahrenheit on average, that was especially harmful to birth weight and it also curtailed gestation,” Ramachandran said.

As the world grows hotter and dryer, dozens of research teams are trying to prepare for the long-term health impacts.

A new study out of the Massachusetts Institute of Technology looked at weather reports and data from the U.S. Centers for Disease Control and Prevention over a 12-year period. Researchers found that an increase in average monthly temperature from around 70 degrees to more than 85 degrees was associated with an increase in the probability of self-reported mental health issues.

A different MIT study found more instances of fatal car crashes and food poisoning on hot days, citing a slump in public safety worker productivity when temperatures are high.

Ramachandran said her team’s work is ongoing, and will soon explore how water policy in the aftermath of the California drought impacts the climate-health relationship, including how access to clean water affects children’s developing brains.